Tuesday, December 17, 2013

With over 300 described species, the
chromodorid nudibranchs are one of the most species rich families of
gastropods.Their bright colors and
interesting morphology appeal to underwater photographers and scientists
alike.Their aposematic coloration has
also drawn the attention of scientists interested in natural products
chemistry.In spite of their
conspicuousness and appeal, there has been no comprehensive, well-supported
phylogeny of the chromodorid nudibranchs.This hinders progress being made in other biological disciplines where
one may assume that species from the same genus represent a monophyletic
group.In a recent issue of PlosOne, Rebecca Johnson and Terrence
Gosliner set out to remedy this situation and generate a phylogeny of the
chromodorid nudibranchs and present a classification that accurately reflects
the evolutionary history of the group.

In the paper entitled “Traditional
Taxonomic Groupings Mask Evolutionary History: A Molecular Phylogeny and New
Classification of the Chromodorid Nudibranchs,” Johnson and Gosliner
assembled the most comprehensive dataset to date including 244 specimens (142
new), representing 157 species (106 new) chromodorid species and several other
taxa.They used two mitochondrial genes
(16S rRNA and cytochrome oxidase I) to reconstruct a phylogeny of the group.The results revealed that currently
recognized genera were either polyphyletic or nested within another genus
rendering the other genus paraphyletic.Extensive homoplasy of morphological attributes that have been thought
to be synapomorphies for a particular genus seems to be the cause of the
unnatural groupings that have been recognized in the past.This pattern of polyphyly of recognized
genera has been observed for unionid bivalves as well (Campbell et al.,
2004).

One appealing outcome of the study by Johnson
and Gosliner (2012) is that they took the time to propose a new classification
of the group based on their findings.As
Johnson and Gosliner (2012) indicate, “the translation of phylogenetic
hypotheses into classifications is the best way to communicate results to a
larger community” and “communicating these new hypotheses is one of the main contributions
systematics can make to the scientific community.”Their study now provides a sound phylogenetic
framework from which morphological, chemical and behavioral attributes can be
examined.

Tuesday, November 26, 2013

Terrestrial gastropods are often a major
component of various terrestrial ecosystems.It is thought that litter-dwelling terrestrial gastropods contribute to
the cycling of nutrients either directly or indirectly through metabolism and
modifying habitat to enhance micro-arthropod or microbial activity,
respectively.However, their role in
ecosystem processes is poorly known particularly in tropical forests.In a recent issue of Biotropica, Wallace M. Meyer III, Rebecca Ostertag, and Robert H.
Cowie shed some light on this very issue in a paper entitled “Influence
of Terrestrial Molluscs on Litter Decomposition and Nutrient Release in a
Hawaiian Rain Forest.”Meyer et al. (2013) used a
field mesocosm approach to examine (1) whether the presence of terrestrial
gastropod species increased rates of leaf litter decomposition, (2) whether different
terrestrial gastropod species influence the rates of nutrient release differently,
and (3) whether terrestrial gastropods facilitate recruitment of
mesoinvertebrates.The results of the
experiments showed that the presence of gastropods increased litter
decomposition rates and that the highest decomposition rates were those with the
greatest gastropod biomass.Furthermore,
although there were differences in the rates of release of some nutrients among
treatments, the different gastropod species appeared to influence nutrient
release in a similar way.Finally, there
was no evidence that terrestrial gastropods facilitated mesoinvertebrate
recruitment.

The authors have shown empirically that
terrestrial gastropods can play a major role in litter decomposition.One interesting aspect of the study is that
it was done using the five most abundant species of gastropods in the Hawaiian
rain forest: the native Succinea cepulla
and four non-native species (Arion
intermedius, Deroceras leave, Oxychilus alliarius, and Limax maximus).The native species had the lowest density
among the gastropods studies and is comparatively rare.Indeed, Hawaii presents a particularly
compelling case because some 65-90 percent of the 750+ species (over 99%
endemic) are now considered extinct (Solem, 1990; Cowie et al. 1995; Cowie,
2001; Lydeard et al., 2004) so there is the distinct possibility that invasive
gastropod species are now conducting important ecological processes that were
once carried out by native species and potentially benefitting otherwise native
ecosystems.Regrettably, important
information is lacking to fully address this issue such as species richness and
densities in historical, native communities.

Friday, October 25, 2013

The January 2013 issue of American Malacological Bulletin included
eight papers from 11 presentations from the James H. Lee symposium, “Great
Unanswered Questions in Malacology,” which was held at the 77th
Annual American Malacological Society meeting in Pittsburgh, Pennsylvania, July
23-27 2011.The organizers, Timothy
Pearce and Charles Sturm, introduced each paper (Pearce and Sturm, 2013).I highlighted one paper previously in my
April blog posting, but one that I would like to highlight further as the topic
for this blog posting is entitled “Recent
advances and unanswered questions in deep molluscan phylogenetics” by Kevin
M. Kocot (Kocot, 2013).Kocot provides a
terrific brief review of the leading hypotheses of molluscan phylogeny that
have been proposed based on morphological and sequence data such as nuclear
small subunit (SSU or 18S) and large subunit (LSU or 28S) ribosomal gene
sequences.Many of these hypotheses have
been debated about over many years with each hypothesis having a leading
advocate or group of advocates supporting them.Regrettably, molecular sequence data, which often provides useful data when
morphology conflicts offered little information to resolve any of the conflicts
and often resulted in bizarre findings such as the lack of monophyly of the
Bivalvia and Gastropoda or a paraphyletic Mollusca. Recently, with the development of
phylogenomics, large amounts of nuclear protein-coding gene data derived from
genomes and transcriptome data instead of PCR to amplify targeted gene
fragments has been generated and found useful in examining the relationships of
animals.In 2011, two papers were
published applying phylogenomics to the test of examining deep molluscan
relationships (Kocot et al. 2011, Smith et al., 2011) and one examined
PCR-amplified regions of seven genes in a target-gene approach (Vinther et al.
2011).

A consensus tree based on the findings of the
three studies was provided as follows:(((Gastropoda,
Bivalvia, Scaphopoda)(Cephalopoda, Monoplacophora))(Polyplacophora,(Neomeniomorpha,
Chaetodermomorpha))).Unlike some
previous hypotheses, it is evident that the Aplacophora is monophyletic and
sister to Polyplacophora rather than being a paraphyletic grade that was basal
and plesiomorphic.This finding alters
our notion of character states for an hypothetical ancestral mollusk.Also, there is no support for the recognition
of the Cyrtosoma (Gastropoda + Cephalopoda), which alters our notion from a
comparative framework for those interested in neurobiology of Cephalopods,
which may actually be sister to Monoplacophora (although Monoplacophora was
only examined in one of the three studies – Smith et al., 2011).

The fact that there was general agreement
among the three studies is comforting and leads one to think perhaps we are
making progress towards understanding deep phylogenetic relationships of the
Mollusca, but many more molecular studies need to be done and sample sizes
increased to determine whether the consensus tree will stand the test of
time.Also, in addition to molecular
sequence data, as Kocot concludes “more traditional morphological and
developmental studies will undoubtedly continue to improve understanding of
molluscan evolution while simultaneously raising new questions about this
fascinating group of animals.”

Friday, August 16, 2013

Species
have been accidentally or purposely disseminated by humans at an alarming rate
over the past century.Most people have
heard about relatively recent invasions and impacts of zebra mussels,
Africanized bees, kudzu and other non-indigenous, exotic, or non-native species
to name a few.What many people may not
realize is that the true globalization and homogenization of the world’s biota
began in 1492 after the “discovery” of the New World by Columbus.After Columbus, ecosystems met and mixed in
an exchange Alfred Crosby referred to as the Columbian Exchange (Mann, 2011).The “exchange took corn (maize) to Africa and
sweet potatoes to East Asia, horses and apples to the Americas, and rhubarb and
eucalyptus to Europe- and also swapped about a host of less-familiar organisms
like insects, grasses, bacteria, and viruses” (Mann, 2011).The Columbian exchange had a profound effect
on the natural ecosystems and landscapes around the world.Exotic species became major staple crops that
most people do not realize are non-native.They are here.We grow them, we
eat them or pet them or ride them.Even
fewer people are aware that species were introduced accidentally or purposely
thousands of years ago by humans – including terrestrial gastropods!

In June 2013, Adele Grindon and
Angus Davison published a paper in PLOS ONE showing that a peculiar distribution pattern of
Cepaea nemoralis land snails in
Ireland and the Eastern Pyrenees was best explained by transportation by
Mesolithic humans over 8000 years ago.Apparently, there are a number of species including the Kerry slug, the
Pyrenean glass snail and the strawberry tree that are found exclusively in
Ireland and Iberia.This distribution
pattern has been referred to as ‘Lusitanian’ and has defied any single
explanation.Grindon and Davison chose
to study C. nemoralis because on the West coast of Ireland, C. nemoralis
has a large, white-lipped morph that is common and also found in the Pyrenees
suggesting some connection that warrants investigation.The researchers sampled across Europe
including Ireland, Britain, northern Spain, southern France and the Pyrenees
and sequenced two mitochondrial gene fragments (cytochrome oxidase subunit I
(COI) and 16S rRNA) to estimate phylogenies and examine the resultant
patterns.What Grindon and Davison
observed was that individuals from Ireland had a mitochondrial lineage, C, that is shared
with Central and Eastern Pyrenean populations. This lineage was absent in most other parts of
Europe with minor exceptions.The
authors propose that the best explanation for the disjunct distribution pattern
is a single historic long distance dispersal event between the Pyrenees and
Ireland.The species has apparently been
a food source in the Pyrenees, so may have been transported live to serve as a
source of food.Grindon and Davison’s
paper is not the first documentation of likely human-mediated dispersal of
terrestrial gastropods.Jesse et al. (2011) hypothesized that Neolithic expansion in
the western Mediterranean resulted in the expansion of the range of Tudorella sulcata s. str. and Lee et al. (2007) hypothesized pre-historic inter-island
introductions of an endemic Pacific island tree snail, Partula hyalina.It is
certain that as other unusual distribution patterns are examined other ancient
human-mediated dispersal events will be discovered and shed light on the
history of world’s biota including gastropods.

LITERATURE CITED

Grindon, A. J. and A. Davison.2011.Irish Cepaea nemoralis land snails have a cryptic Franco-Iberian origin
that is most easily explained by the movements of Mesolithic humans.PLOS ONE 8(6):1-7.

About Me

I am the Chair of the Department of Biology and Chemistry at Morehead State University and have been studying the systematic relationships of freshwater mollusks for over 20 years. I am currently managing the American Malacological Society's Molluscan Musings which features various topics on systematics and biodiversity of mollusks. I will be contributing and coordinating activity on these blogs including inviting guest contributors. Thanks for checking it out.